STAGES OF ANIMAL DEVELOPMENT Ch. 42
In all sexually reproducing animals a new individual begins
life as a zygote, the diploid cell that forms at fertilization.
Development from a zygote into an adult typically proceeds
through a series of stages that we all have in common.
 Fertilization occurs when sperm penetrates an egg.
 Mitotic cell divisions create a ball of cells called a
blastula, each cell obtains a different part of the
original cytoplasm, due to cytoplasmic localization.
o The blastula enclose a cavity called the blastocoel
filled with their secretions.
 Gastrulation occurs as the ball of cells cups inwards
creating the primary tissue layers.
o The primary tissues called germ layers consist of….
 Ectoderm – outer layer, gives rise to nervous
tissue.
 Endoderm – inner layer, gives rise to respiratory
tract and gut lining.
 Mesoderm – middle layer, gives rise to muscles,
connective tissue, circulatory system.
 Organs form as a result of tissue interactions, cells
move change shape and apoptosis occurs.
o Many organs are formed from more than one germ
layer.
 Organs grow in size gradually assuming specialized
function.
Now let’s add some detail!
FERTILIZATION –
Sperm consisting of paternal DNA penetrates an egg. The
egg contains special yolk proteins, mRNAs, tRNAs and
ribosomes for creating proteins essential to early
development.
Cytoplasmic localization occurs in all oocytes. The
components of the cytoplasm are not distributed evenly in a
yolk rich egg. The vegetal pole has most of the yolk, the
animal: is more pigmented due to the cell cortex, a
cytoplasmic region just beneath the plasma membrane.
After fertilization, a cell cortex rotates revealing a gray
crescent opposite the site of sperm penetration. The 1st
cleavage splits the gray crescent in 2, creating a left and
right side. Cleavage continues, putting different parts of
the cytoplasm into the new cells as a blastomere forms.
Each species has a specific cleavage pattern, coelomates are
divided into the protostomes (1st mouth) and dueterostomes (
2nd mouth)
http://www.cengage.com/biology/book_content/9781111425692_starr_udl13e/animat
ions/PowerPoint_Lectures/chapter42/videos_animations/cytoplasmic_localization.ht
ml
During GASTRULATION cells begin to rearrange themselves,
and the hollow ball begins to turn in on itself. Gray crescent
cytoplasm forms dorsal lip to signal gastrulation. Primary
germ layers form.
Cell differentiation begin as intercellular signals encourage
selected gene expression. Morphogens are signal molecules.
They can act over a long distance influencing cells in a
concentration dependent way. They are produced in one area
of an embryo and diffused outward. Bicoid protein (a type
of morphogen) is a maternal effect gene expressed during
egg production. Where its concentrations are greatest, it
regulates the development of anterior body structures.
Lowest concentrations regulate the development of posterior
body structures.
Another way to look at bicoid protein is that it is a
transcription factor that stimulates the expression of other
master genes which regulate other master genes and so on.
Embryonic tissue can alter the behavior of cells within an
adjacent tissue through the process of embryonic induction.
http://www.cengage.com/biology/book_content/9781111425692_starr_udl13e/animat
ions/PowerPoint_Lectures/chapter42/videos_animations/dorsal_lip.html
After gastrulation vertebrate organ formation begins with
the neural tube, induced by signals from the notochord,
which formed early from mesoderm, overlying ectodermal
cells elongation forming a thick neural plate.
http://www.cengage.com/biology/book_content/9781111425692_starr_udl13e/animat
ions/PowerPoint_Lectures/chapter42/videos_animations/gastrulation_mechanisms_
m.html
Cell elongation results from assembly of microtubules inside
the edges become wedge-shaped actin at one end constricts,
changes cell shape, and causes the edges of the plate to
fold in word forming a neural groove and eventually forming
the neural tube.
Cell migrations are essential part of development. Some of
the ectodermal cells from the neural tube migrate outward
to form neurons, glial cells, peripheral nervous system and
melanocytes in the skin. Cells travel by inching along an
amoeba like fashion, but I think it looks more like an
inchworm.
As development and specialization continue programmed cell
death must occur in response to chemical signals, helping
sculpt body parts. This process is called apoptosis.
AN EVOLUTIONARY VIEW OF DEVELOPMENT
Through studies of many animals in general model for
development has been formatted.
1st – cytoplasmic localization causes localized expression of
master genes in the zygote. Products of the master genes
defuse outward, creating a gradient and a head to tail, and
dorsal to ventral axis for the developing embryo.
2nd – cells in the embryo activate other master genes,
products of these genes become distributed ingredients
which affect other genes and so on (bicoid proteins)
3rd – positional information effects suppression of homeotic
genes.
Phylogenetic constraints on body plan are imposed by
interactions among genes that regulate development. Once
master genes evolved their interactions determined the basic
body form. Mutations in these master genes are often
lethal.
In many vertebrates paired bones and muscles exist in a
head to tail orientation. During development blocks of cells
called Somites are visible on either side of the embryos
neural tube, peaceful way to develop into bones in skeletal
muscles. Somites are often associated with segmented body
plans and vary in number. Again mutations within somites are
often lethal.
Ok lets have a BLAST!!
The blast problem………
Blastula----a hollow ball of cells, with a fluid filled cavity,
cleavage is considered over at this stage.
Blastocoel-----the fluid fill cavity of the blastula
Blastomere-----one of the cells of the blastula
Blastocyst---is a blastula BUT later in development, it has
an inner cell mass that eventually becomes the embryo. OH!
It is also only a Mammal thing. Not lower animals. (the
outter ball of cells will become the placenta)